1. Field of the Invention
This invention relates to a semiconductor light-detecting element, preferably usable as a photodiode or the like.
2. Related Art Statement
Recently, a semiconductor light-detecting element such as a photo-diode is built as a photodetector in daily necessities such as a television set, a video deck, a stereo set or an air conditioner as well as an automatic controller, an optical measuring instrument and an optical communication system. As such a semiconductor light-detecting element, a Ga-based nitride semiconductor is employed because the semiconductor material has a large direct band-gap and the band-gap can be controlled freely by adjusting the composition.
FIG. 1 is a structural view showing a conventional so-called PIN-type semiconductor light-detecting element.
In a semiconductor light-detecting element 10 shown in FIG. 1, on a substrate 1 made of a given single crystal such as sapphire, ZnO, SiC, Si, GaAs or GaN are formed a buffer layer 6 made of AlN, an underlayer 2 made of i-GaN, an n-type conductive layer 3 made of n-AlGaN, a light-detecting layer 4 made of i-AlGaN and a p-type conductive layer 5 made of p-GaN. The n-type conductive layer 3 is partially removed and exposed, and an n-type electrode 7 is formed of Al/Ti on the exposed surface of the n-type conductive layer 3, and a p-type electrode 8 is formed of Au/Ni on the p-type conductive layer 5.
When a light with a wavelength of a cut-off wavelength or below to be detected is introduced into the semiconductor light-detecting element 10, the light-detecting layer 5 is excited by the introduced light, and thus, a given current is flown in a given electric circuit including the semiconductor light-detecting element 10 via the n-type electrode 7 and the p-type electrode 8. Then, the introduced light is detected by measuring the current flown in the electric circuit.
In the semiconductor light-detectine element 10 shown in FIG. 1, the buffer layer 6 has the function to compensate the difference in lattice constant between the substrate 1 and the underlayer 2 and thus, realize the epitaxial growth of the light-detecting layer 4. In this point of view, the buffer layer 6 is made in amorphous at a lower temperature within 500-700xc2x0 C., with disregard to the crystal quality.
Therefore, a relatively large amount of dislocation is created in the buffer layer 6, and then, partially propagated as threading dislocations in the underlayer 2, the n-type conductive layer 3, the light-detecting layer 4 and the p-type conductive layer 5. As a result, a large amount of dislocation results in being created in these layers at a dislocation density of 1010/cm2 or over, and thus, the crystal qualities of these layers are deteriorated. This trend becomes conspicuous particularly if the semiconductor light-detecting element 10, that is, the n-type conductive layer 3 and the light-detecting layer 4 includes relatively large amounts of Al.
If the semiconductor light-detecting element 10 is made of such layers with large dislocation densities and thus low crystal qualities, a relatively large dark current is flown through the dislocations created in each layer. Therefore, when the introduced light is detected by measuring the current flown in the electric circuit, the measurement error is increased due to the dark current, and the S/N ratio or the detecting sensitivity of the introduced light is remarkably deteriorated.
It is an object of the present invention to provide a semiconductor light-detecting element which can reduce a dark current and thus, realize a higher detecting sensitivity.
For achieving the above object, this invention relates to a semi-conductor light-detecting element comprising a given substrate, an underlayer and a light-detecting element structure which are formed on the substrate in turn,
the underlayer being made of a nitride semiconductor including Al element with a dislocation density of 1011/cm2 or below,
the light-detecting element structure being made of a nitride semiconductor layer group including Al element at a larger content than the nitride semi-conductor making the underlayer with a dislocation density of 1010/cm2 or below.
The inventors had intensely studied to achieve the above object. Then, since the above-mentioned dark current is originated from many dislocations in the lower crystal quality layers constructing the semiconductor light-detecting element, they made an attempt to reduce the dislocations in the layers and improve the crystal qualities of the layers.
As mentioned above, the dislocations in the layers constructing the semiconductor light-detecting element are originated from the buffer layer with a lower crystal quality. Moreover, as the Al content of the n-type conductive layer 3 is increased, a large amount of dislocation is created in the conductive layer 3 because many dislocations are created at the boundary surface between the underlayer 2 and the conductive layer 3, and some cracks may be created in the conductive layer 3. This phenomenon is originated from a tensile stress in the conductive layer 3 due to smaller in-plane lattice constant in Al-richer III-nitride material.
In this point of view, the inventors made various attempts for the underlayer as well as the buffer layer. As is apparent from FIG. 1, the conventional semiconductor light-detecting element is constructed of the conductive layers and light-detecting layer made of Ga-based nitride semiconductor. So, it is natural that the underlayer is also made of Ga-based nitride semiconductor, for example, GaN. As a result, a mismatch in lattice constant for the substrate made of such as sapphire single crystal is created, and thus, a buffer layer made at a lower temperature is required to mitigate the mismatch.
However, the inventors paid an attention to the composition of the underlayer and made an attempt to vary the composition. Consequently, they found out that the underlayer is made of Al-based nitride semiconductor which has a lower dislocation density and thus, higher crystal quality. The underlayer can be epitaxially grown directly on the substrate made of e.g., sapphire single crystal without the buffer layer by compensating the difference in lattice constant. In addition, the crystal qualities and thus, the dislocation densities of the conductive layers and the light-detecting layer which are epitaxially grown on the under-layer can be improved.
As a result, the dislocation densities of the respective semiconductor layers constructing the semiconductor light-detecting layer are reduced and thus, the dark current can be effectively repressed. Therefore, the error of the measured current value flown in the electric circuit due to the dark current can be reduced, and thus, the detecting sensitivity of the semiconductor light-detecting element can be improved.